This invention is directed to the use of a nonflammable, nontoxic, chlorine free refrigerant mixture for use in very low temperature refrigeration systems.
Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial settings. In particular, low-temperature refrigeration systems currently provide essential industrial functions in biomedical applications, cryoelectronics, coating operations, and semiconductor manufacturing applications.
Providing refrigeration at temperatures below 223 K (−50 C.) have many important applications, especially in industrial manufacturing and test applications. This invention relates to refrigeration systems which provide refrigeration at temperatures between 223 K and 73 K (−50 C. and −200 C.). The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this application the term “very low” or “very low temperature” will be used to mean the temperature range of 223 K and 73 K (−50 C. and −200 C.). In many manufacturing processes conducted under vacuum conditions, and integrated with a very low temperature refrigeration system, rapid heating is required for some elements. This heating process is a defrost cycle. The heating warms the evaporator and connecting refrigerant lines to room temperature. This enables these parts of the system to be accessed and vented to atmosphere without causing condensation of moisture from the air on these parts. The longer the overall defrost cycle and subsequent resumption of producing very low temperature temperatures, the lower the throughput of the manufacturing system. Enabling a quick defrost and a quick resumption of the cooling of the cryosurface (evaporator) in the vacuum chamber is beneficial to increase the throughput of the vacuum process.
There are many vacuum processes which have the need for such very low temperature cooling. The chief use is to provide water vapor cryopumping for vacuum systems. The very low temperature surface captures and holds water vapor molecules at a much higher rate than they are released. The net effect is to quickly and significantly lower the chamber's water vapor partial pressure. This process of water vapor cryopumping is very useful for many physical vapor deposition processes in the vacuum coating industry for electronic storage media, optical reflectors, metallized parts, semiconductor devices, etc. This process is also used for remove moisture from food products in freeze drying operations.
Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces being cooled to lower temperatures than the panels. Yet another application is the removal of heat from objects being manufactured. In some applications the object is an aluminum disc for a computer hard drive, a silicon wafer for the manufacture of a semiconductor device, or the material such as glass or plastic for a flat panel display. In these cases the very low temperature provides a means for removing heat from these objects more rapidly, even though the object's final temperature at the end of the process step may be higher than room temperature. Further, some applications involving, hard disc drive media, silicon wafers, or flat panel display material, or other substrates, involve the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface like a platen is the typical means of removing heat from such objects. In all these cases an interface between the refrigeration system and the object to be cooled is proceeding in the evaporator where the refrigerant is removing heat from the object at very low temperatures.
Still other applications of very low temperatures include the storage of biological fluids and tissues, control of reaction rates for chemical processes and pharmaceutical processes.
Conventional refrigeration systems have historically utilized chlorinated refrigerants, which have been determined to be detrimental to the environment and are known to contribute to ozone depletion. Thus, increasingly restrictive environmental regulations have driven the refrigeration industry away from chlorinated fluorocarbons (CFCs) to hydrochloro fluorocarbons (HCFCs). Provisions of the Montreal Protocol require a phase out of HCFC's and a European Union law bans the use of HCFCs in refrigeration systems as of Jan. 1, 2001. Therefore the development of an alternate refrigerant mixture is required. Hydroflurocarbon (HFC) refrigerants are good candidates which are nonflammable, have low toxicity and are commercially available. The use of HFC's in commercial and residential applications is now well known. However, these applications do not require the typical HFC refrigerants to be used at very low temperature. Therefore their performance and behavior in a mixture at low temperature is not known.
When selecting replacement refrigerants, the use of nonflammable, nontoxic (permissible exposure limit greater than 400 ppm) is preferred.
Prior art very low temperature systems used flammable components to manage oil. The oils used in very low temperature systems using chlorinated refrigerants had good miscibility with the warmer boiling components which are capable of being liquefied at room temperature when pressurized. Colder boiling HFC refrigerants such as R-23 are not miscible with these oils and do not readily liquefy until colder parts of the refrigeration process. This immiscibility causes the compressor oil to separate and freezeout which leads to system failure due to blocked tubes, strainers, valves or throttle devices. To provide miscibility at these lower temperatures, ethane was added to the refrigerant mixture. Unfortunately, ethane is flammable and can limit customer acceptance and can invoke additional requirements for system controls, installation requirements and cost. Therefore, elimination of any flammable component is preferred.
In addition, use of a toxic refrigerant can limit customer acceptance and can invoke additional requirements for system controls, installation requirements and cost. A permissible exposure limit (PEL) is the maximum amount of concentration of a chemical that a worker may be exposed to under OSHA regulations. In the case of mixed refrigerants, a PEL of any component below 400 ppm is considered toxic and poses a health risk to any individual, such as a service technician, that may be exposed to the refrigerant. Therefore it is beneficial to use a refrigerant whose components have a PEL that is greater than 400 ppm.
Another requirement is to develop a mixture of refrigerants that will not freezeout from the refrigerant mixture. A “freezeout” condition in a refrigeration system is when one or more refrigerant components, or the compressor oil, becomes solid or extremely viscous to the point where it does not flow. During normal operation of a refrigeration system, the suction pressure decreases as the temperature decreases. If a freezeout condition occurs the suction pressure tends to drop even further creating positive feedback and further reducing the temperature, causing even more freezeout. What is needed is a way to prevent freezeout in an MR refrigeration system. HFC refrigerants available have warmer freezing points than the HCFC and CFC refrigerants that they replace. Since these refrigerants are rather new and since their use at very low temperatures is uncommon there is no body of information that can predict the freezeout behavior of mixtures containing these new refrigerants.
Another challenge when using hydrofluorocarbons (HFCs) is that these refrigerants are immiscible in alkylbenzene oil and therefore, a polyolester (POE) (1998 ASHRAE Refrigeration Handbook, chapter 7, page 7.4, American Society of Heating, Refrigeration and Air Conditioning Engineers) compressor oil is used to be compatible with the HFC refrigerants. Selection of the appropriate oil is essential for very low temperature systems because the oil must not only provide good compressor lubrication, they also must not separate and freezeout from the refrigerant at very low temperature.
Typically, in the refrigeration industry, a change in refrigerants requires a change in hardware elements such as the compressor or valves. As a result, a refrigerant change can cause expensive equipment retrofit and associated down time. What is needed is a way to use existing refrigeration equipment in combination with the recently developed HFC mixed refrigerants that are compatible with the existing hardware and materials. This is further complicated by the fact that very low temperature systems must operate in several different modes. Even the start up process on these systems can be challenging since many of the refrigerants that are liquid during steady state operation are in a gaseous state when the system is at room temperature. Further, severe operational changes such as providing rapid defrost require proper refrigerant blending for the system to operate without exceeding limits on operating temperatures or pressures. The individual developed blends in accordance with the invention are shown in Table I (FIG. 1) and indicated as Blend A, Blend B, etc. Also shown in the table are the model numbers of developed commercial products IGC Polycold Systems, Inc., San Rafael, Calif. which use these blends.
For example, a prior art refrigeration unit, used a mixture, containing R-123, R-22, R-23, R-170, R-14, and argon, which mixture has been successfully replaced with Blend A (Table I) to achieve the goal of providing equivalent refrigerant performance without using HCFC's and without using flammable or toxic refrigerants.
Further, in accordance with the invention, another component may be added to the above compositions provided that the ratios of the listed components (Table I) remain in the same proportions relative to each other.